I cannot figure out how to pan them 100% left or right. It still keeps some sound in the other channel.
I had the same issue with Audacity.
In Audacity, you'd expand the vertical size of the channel you want to pan, so that you can see all of the menus on the left side of it.
Once it's expanded visually, you can pan full left or full right for mono channels, or move the pan control of a stereo pair, though I don't think I ever do that.